Problem: Jessica is 4 times as old as Nadia and is also 9 years older than Nadia. How old is Jessica?
Solution: We can use the given information to write down two equations that describe the ages of Jessica and Nadia. Let Jessica's current age be $j$ and Nadia's current age be $n$ $j = 4n$ $j = n + 9$ Now we have two independent equations, and we can solve for our two unknowns. One way to solve for $j$ is to solve the second equation for $n$ and substitute that value into the first equation. Solving our second equation for $n$ , we get: $n = j - 9$ . Substituting this into our first equation, we get the equation: $j = 4$ $(j - 9)$ which combines the information about $j$ from both of our original equations. Simplifying the right side of this equation, we get: $j = 4j - 36$ Solving for $j$ , we get: $3 j = 36$ $j = 12$.